The present invention relates generally to electronic devices whose functional length scales are measured in nanometers, and, more particularly, to transistors based on crossed nanometer-scale wires, at least one of which is a modulation-doped semiconductor, joined by functionalized groups at the intersecting junctions.
The silicon (Si) integrated circuit (IC) has dominated electronics and has helped it grow to become one of the world""s largest and most critical industries over the past thirty-five years. However, because of a combination of physical and economic NOMIC reasons, the miniaturization that has accompanied the growth of Si ICs is reaching its limit. The present scale of devices is on the order of tenths of micrometers. New solutions are being proposed to take electronics to ever smaller levels; such current solutions are directed to constructing nanometer scale devices.
Prior proposed solutions to the problem of constructing nanometer scale devices have involved (1) the utilization of extremely fine scale lithography using X-rays, electrons, ions, scanning probes, or stamping to define the device components; (2) direct writing of the device components by electrons, ions, or scanning probes; or (3) the direct chemical synthesis and linking of components with covalent bonds. The major problem with (1) is that the wafer on which the devices are built must be aligned to within a small fraction of the size of the device features in at least two dimensions for several successive stages of lithography, followed by etching or deposition to build the devices. This level of control does not scale well as device sizes are reduced to nanometer scale dimensions. It becomes extremely expensive to implement as devices are scaled down to nanometer scale dimensions. The major problem with (2) is that it is a serial process, and direct writing a wafer full of complex devices, each containing trillions of components, could well require many years. Finally, the problem with (3) is that high information content molecules are typically macromolecular structures such as proteins and DNA, and both have extremely complex and, to date, unpredictable secondary and tertiary structures that cause them to twist into helices, fold into sheets, and form other complex 3D structures that will have a significant and usually deleterious effect on their desired electrical properties as well as make interfacing them to the outside world impossible.
The present inventors have developed new approaches to nanometer-scale devices, comprising crossed nano-scale wires that are joined at their intersecting junctions with bi-stable molecules, as disclosed and claimed in application Ser. No. 09/282,048, filed on even date herewith, now U.S. Pat. No. 6,459,095, issued Oct. 1, 2002. Wires, such as silicon, carbon and/or metal, are formed in two-dimensional arrays. A bi-stable molecule, such as rotaxane or pseudo-rotaxane, is formed at each intersection of a pair of wires. The bi-stable molecule is switchable between two states upon application of a voltage along a selected pair of wires.
Prior solutions to the problem of constructing a nanometer scale transistor (a three-terminal device with gain) involve the precise positioning of three or four components within a nanometer. A proposed prior solution is to position a quantum dot between two wires, which act as the source and drain of the transistor, in tunneling contact with the quantum dot (this is known as a single-electron transistor, or SET, and was originally proposed by K. Likharev); see, e.g., K. K. Likharev, xe2x80x9cCorrelated discrete transfer of single electrons in ultrasmall tunnel junctionsxe2x80x9d, IBM Journal of Research and Development, Vol. 32, pp. 144-158 (Jan. 1998). A third wire is positioned in capacitive contact with the dot, which is the gate. The voltage on the gate changes the energy levels in the quantum dot, which creates a coulomb blockade to current flowing from the source to the drain.
The problem with the foregoing construction is that a total of three wires plus a quantum dot have to be precisely positioned. This precision of placement must clearly be better than the sizes of the components being placed, which are in the order of nanometers. Such devices have been fabricated using conventional lithography, in which case they are so large they must be operated at very low temperatures (near absolute zero). They have also been fabricated using electron beam lithography and utilizing scanning probe microscopes as direct-write tools.
Thus, there remains a need to provide a nanoscale transistor that affords the advantages of prior art devices, while avoiding most, if not all, their disadvantages.
In accordance with the present invention, a molecular wire transistor is provided, comprising a pair of crossed wires, with at least one of the wires comprising a modulation-doped semiconductor material. A first semiconductor wire is of a first conductivity type and a second wire is provided with either Lewis acid functional groups or Lewis base functional groups to create a region of modulation doping of a second and opposite conductivity type in the junction.
If both functionalized wires are doped semiconductor, such as silicon, one is P-doped and the other is N-doped. One wire of a given doping comprises the xe2x80x9cemitterxe2x80x9d and xe2x80x9ccollectorxe2x80x9d portions and the other wire induces the xe2x80x9cbasexe2x80x9d function in the first wire containing the emitter and collector at the junction where the wires cross, between the emitter and collector portions. The xe2x80x9cbasexe2x80x9d region is always modulationoped in the present invention. Both PNP and NPN transistors that are analogous to bipolar transistors may be formed in this fashion.
One functionalized wire may comprise doped semiconductor, such as silicon, and the other functionalized wire may comprise a metal. Here, the doped semiconductor wire comprises the xe2x80x9csourcexe2x80x9d and xe2x80x9cdrainxe2x80x9d and the metal wire induces the xe2x80x9cgatexe2x80x9d function on the doped semiconductor wire where the wires cross, between the source and drain, to form a field effect transistor. The xe2x80x9cgatexe2x80x9d region is always modulation-doped in the present invention. Both N-channel and P-channel transistors that are analogous to field effect transistors may be formed in this fashion.
Further, a molecular memory effect (state change) may be incorporated into the coatings that dope the wires. By choosing the molecule that does the doping of the gate to have two distinct oxidation-reduction (redox) states, a conductive state and a relatively insulating state with a large I-V hysteresis separating the two, it then becomes possible to have a special type of transistor. If the state change is set, then a transistor is formed, but if the state change is not set, then either an open or closed switch is formed. Whether a switch is open or closed depends on the state of a wire coatingxe2x80x94whether it is oxidize or reduced.
A second version of molecular memory effect (molecular configuration bit) is to use a bi-stable molecule such that if the memory bit is set, the semiconductor wire is N-doped along its entire length and conducts. But if the memory bit is reset by applying a sufficient voltage difference between the two wires, then the molecule induces a P region through modulation doping, and the semiconductor wire will not conduct across the region where the wires cross. The N and P regions can be interchanged to make another form of the invention.
The present invention enables the construction of transistors on a nanometer scale, which are self-aligned and modulation-doped. It is very difficult at a nanometer scale to position and align the three components of a bipolar transistor (emitter, base collector) or a field effect transistor (source, gate, and drain). It is also very difficult at a length scale defined in angstroms to control the exact properties of the doping that defines the electrical properties of the semiconductor.
The present invention allows transistors to be formed with a size on the order of tens of nanometers to a few nanometers. By choosing the molecules which form the doping layer, it is possible to build transistors with a wide variety of specifically desired electrical properties. The inclusion of an electrically settable memory bit, through an electrochemical reaction with a large hysteresis loop in its I-V characteristic, as part of the molecules that define the gate region allows a new and useful function to be added to transistors. Another advantage of this invention is that the same technology allows one to choose to build both NPN and PNP bipolar transistors as well as P-channel and N-channel FETs.